This disclosure relates to a system and a method for the operation of a furnace that is operated with at least one coal pulverizer.
Systems and methods for the operation of a furnace that is operated with at least one coal pulverizer are known. Thus, for example, in the document “Kraftwerkstechnik zur Nutzung fossiler, regenerativer und nuklearer Energiequellen” [“Power plant engineering for utilization of fossil, regenerative and nuclear energy sources”] by Karl Strauss, Springer Verlag, 1994, a system and a method of this type are shown. In Chapter “4.3.2.2 Staubfeuerungen” [“Coal Dust Firings”] from the document mentioned above, the detailed mode of operation of a furnace with at least one coal pulverizer can be found, whereby the block diagram that is shown illustrates a coal pulverizer with the associated incoming and outgoing lines of primary air, raw coal, coal dust, etc. According to this, the coal needed for firing in the furnace is pulverized into coal dust and dried in at least one coal pulverizer. To dry the coal inside the mill, as well as for the combustion of the coal dust in the furnace, preheated air, i.e., primary air, is supplied that is generated through the exchange of heat in an air preheater (“Luvo”) of the fresh air that is being drawn in with the hot exhaust gas, i.e., flue gas, from the combustion of the coal in the furnace. In that regard, the majority of the fresh air that is drawn in is directed through the air preheater and thereby heated, and the remaining portion of fresh air that is drawn in is diverted upstream of the air preheater and is mixed unheated, i.e., as cold air, back into the heated portion of the fresh or primary air downstream of the air preheater. To ensure that the coal pulverizer is charged with primary air of a certain temperature range, the primary air temperature can be adjusted by admixing a certain amount of cold primary air into the hot primary air by appropriately adjusting the flow control butterfly valves in the incoming lines of the heated and cold primary air. The heated, i.e., hot primary air which, as already mentioned above, is utilized for drying the pulverized coal dust, functions within the mill, i.e., coal pulverizer, and downstream of the mill to the coal dust burners of the furnace, as carrier air as well, since it entrains or carries the coal dust particles in suspension.
Since operation with coal dust inside a furnace or coal dust furnace holds certain dangers, the regulations TRD 413 (Technical Regulations for Steam Boilers) and European Standard EN 12952, Part 9, provide for specific safety measures. They can be found, for example, in the document “Explosionsschutzmassnahmen für Kohlenstaubfeuerungen in Kraftwerken” [“Explosion Protection Measures for Coal Dust Firings in Power Plants”] by F. Arens-Fischer, VGB Kraftwerkstechnik [Technical Association of Large Power Plant Operators, Power Plant Engineering] 72 (1992), Number 6. There, in FIG. 1 and its associated description, it is explained that certain requirements of TRD 413 lead to the installation of safety quick-acting butterfly valves in the carrier air lines, i.e., the mill (primary) air lines. Within a safe period of time, these safety quick-acting butterfly valves interrupt the feeding of coal dust into the combustion chamber, since they cut off the feeding of carrier or primary air to the mill, and thus no more air that could carry coal dust flows through the coal pulverizer in the direction of the coal dust burners, i.e., the combustion chamber. Usually, these safety quick-acting butterfly valves or safety butterfly valves are designed as swiveling butterfly valves that are opened by means of pneumatic, hydraulic or electric actuators and closed by means of springs or weights (the so-called safety position). Lamellar seals are usually used to keep the leakage of the safety quick-acting butterfly valves low, and the safety butterfly valves are located in channel pockets in order to prevent wear on their wings.
TRD 413 further provides that for repair and servicing purposes, shut-off arrangements be provided ahead of and behind the coal pulverizer if work is to be done on the coal pulverizer during operation of the coal dust furnace or the boiler plant. For shut-off ahead of the coal pulverizer, i.e., upstream of the coal pulverizer when seen in the direction of flow of the primary air, the existing quick-acting or safety butterfly valve is used as the shut-off valve in order to avoid having to install a second butterfly valve ahead of the coal pulverizer. However, it has been shown that by doing this, despite the high-quality, expensive sealing system in the safety butterfly valve, hot primary air in the form of leakage air gets past the safety butterfly valve and flows into the coal pulverizer. As a result of the flow of hot primary air into the coal pulverizer, however, it is not safe to perform repair or servicing work inside the coal pulverizer. To remedy this, large covers on the primary air line of doors on the coal pulverizer have to be opened in order to keep temperatures inside the coal pulverizer tolerable for the repair personnel or servicing personnel. This requires an additional, time-consuming installation effort, whereby the working conditions inside the coal pulverizer are sometimes not significantly improved.
In the design of the latest generation of power plants (>800 MW of electricity), new coal pulverizers with very large dimensions are required for the furnaces, i.e., coal dust furnaces, of the power plants, which of necessity require primary air lines and safety or shut-off butterfly valves with larger dimensions. It has been shown in that regard that the own weight of the enlarged construction of the safety butterfly valve designed as a swiveling butterfly valve increases not linearly, but progressively. The same holds true for the increase of the torque required for actuating the swiveling butterfly valve by the pneumatic, hydraulic or electrical actuator, since when the swiveling butterfly valve is opened, the counterpressure resulting from the primary air present at the swiveling butterfly valve has to be overcome. Because of its increased area, an enlarged swiveling butterfly valve thus results in greater counterpressure from the primary air, thus substantially increasing torque required for actuating the swiveling butterfly valve. Compared with the safety butterfly valves that have been common and in use up to now, disproportionally growing costs are required for the production or acquisition of the safety butterfly valve and for the required components, particularly the actuating drive for the safety butterfly valve.
In contrast to the regulations TRD 413 and EN 12952, Part 9, the American regulation NFPA 8503, Standard for Pulverized Fuel Systems, provides that the quick-acting butterfly valve required for emergency situations to shut off the flow of primary or carrier air flow is not arranged upstream of the coal pulverizer, but downstream of the coal pulverizer instead. In the event of necessary repair or servicing on a coal pulverizer, this regulation provides butterfly shut-off valves upstream of the coal pulverizer. However, it has been shown that depending on the design of the shut-off butterfly valves, they are more or less leakproof, and therefore leakage air can flow through the shut-off butterfly valves and continue on into the coal pulverizer. Safe servicing or repairs inside the coal pulverizer is then no longer assured.